Conventional aluminum aircraft structures typically have inherent lightning protection. An aluminum skin structure has uniform and predictable material properties, including the electrical properties. The electrical conductivity impacts electromagnetic effects (EME) such as lightning, shielding, precipitation static and electrostatic performance. The high conductivity of aluminum provides inherent shielding. Thus currents are conducted on the outside structure, not on internal structures and systems reducing the need for additional protection features such as cable shields or fuel tank sealant to contain sparking Protection against skin puncture by lightning can be provided simply by sizing the aluminum skin thickness. In areas of conventional aircraft structure where the skin is electrically non-conductive, such as radomes and aerodynamic fairings, metal bus bars can be applied to the exterior surface to direct the lightning currents to the aluminum structure. Expanded metal foils, woven wire meshes and interwoven wire forms of lightning protection are often co-cured into a composite as a protective layer to prevent lightning from puncturing the part and/or to provide electromagnetic shielding.
Providing lightning protection for electrically conductive composite material structure, such as carbon fiber reinforced plastic (CFRP), is much more difficult and complicated than for typical aluminum structure. Conductive composites are nonhomogeneous and are considerably less conductive than aluminum. The conductivity of conductive composite material structures such as CFRP panels may therefore be an important parameter to determine the performance of the composite during lightning strike. In another context, the conductivity may be an important parameter to know in the induction heating process involving these structures. Conductivity and thickness also determine shielding characteristics. Different frequencies correspond to different types of shielding.
For a multiply composite material structure, it may be desirable to know the conductivity of the structure in the directions of the ply, transverse to the ply and between plies. Interply conductivity may be particularly important because it can drive lightning performance in terms of edge glow and fastener sparking. For induction heating, knowledge of the interply conductivity may indicate local hot spots.
Techniques currently exist for measuring the interply conductivity of a composite material structure. In one technique, interply conductivity may be measured with current pulse measurements on multiple samples of a specimen structure. This technique requires a direct electrical contact with the specimen and can be time consuming to prepare the samples. In addition, the test itself requires a significant amount of time. Different specimen structures often yield different interply conductivities, but it is not known if this is due to variations in the test specimen, or in the test itself. The current technique requires machining, sanding and plating the specimen structures into small, close-tolerance specimens; and after the structures are tested, they cannot be used again. It is a long and laborious process to characterize the interply conductivity for a new composite material structure. Measured values need to be corrected for contact resistance.
There is also no reliable method to monitor corrosion degradation for some types of specimens when subjected to environmental conditioning. Existing techniques require touching them with a probe, typically after sanding through the resin layer on composites. This can lead to more degradation than if the specimen was left fully intact throughout the entire conditioning. This is especially problematic for salt spray which requires checking specimens at regular intervals as the sanding creates a water path that can accelerate corrosion. Co-curing or installing special features for the probe to touch instead of sanding changes the test article.
Therefore, it may be desirable to have a system and method that takes into account at least some of the issues discussed above, as well as possibly other issues.